Method of making electrodes from fluid coke blends



United States Patent METHOD OF MAKING ELECTRODES FROM FLUID COKE BLENDS Joseph F. Nelson, Westfield, and Brook I. Smith, Elizabeth, N. J., assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Application November 2?, 1954 Serial No. 471,881

2 Claims. (Cl. 106-284) This invention relates to carbon electrodes and the manner of their preparation from mixtures of delayed and fluid coke. More particularly it relates to the preparation of electrodes of this nature which can be utilized for the obtaining of aluminum from its ores.

In the manufacture of aluminum by electrolytic reduction of alumina in a suitable fused bath, the necessary carbon electrodes have usually been manufactured from socalled petroleum coke of relatively high purity.

Petroleum coke had been obtained largely from coking processes such as delayed coking which provide particles of relatively large diameter and densities. Delayed coking is the well known cracking method for the thermal conversion of heavy hydrocarbon oils to lighter fractions and coke. The process employs a reaction or coking chamber designed to accumulate. substantial quantities of coke between cleanings. Two vertical coke drums are generally employed, one of which is decoked while the other is onstream. Temperatures of about 750 to 900 F. are employed.

The calcined, delayed coke product has a high real or particle density, i. e., 2 or higher. The size distribution of the delayed coke particles utilized in electrode manufacture is such that a predominant portion, i. e., about 80 wt. percent, has a diameter in the range of about /2" to 200 mesh with the balance finer than 200 mesh. It has thus been thought that comparatively large particles and high real densities wererequired for satisfactory elec trodes. The principal criteria of these finished electrodes are a minimum compression strength of 4400 pounds per square inch, a minimum real density of about 1.45 and a maximum resistivity of 3 l0 ohm-inch.

One of the problems in using these electrodes is the tendency to dusting, the premature breakdown or shredding of the electrode in the alumina bath. This unduly increases electrode requirements and can result in short circuiting the bath. The dusting is believed to be caused by the selective action of evolved oxygen on the lower density carbonaceous material derived from the binder in the finished electrode as compared to the carbon from the delayed coke.

It has now been found that this difficulty of the carbon electrodes can be overcome by utilizing a mixture of calcined delayed and calcined fluid coke as the charge stock to the electrode fabricating process. The fluid coke is utilized in an amount of from 1 to 50 wt. percent, preferably 20 to 40 wt. percent, based on the total coke charge.

It is surprising that blending the fluid coke with delayed coke accomplishes this result in view of the fact that the formers real density and particle size had been thought to be too low for fabrication, as is, into electrodes. In addition electrodes made exclusively from fluid coke as used herein are of inferior quality. The

electrodes made from the blends, however, are the equals of those made from delayed coke alone in all other requirements and in addition are essentially free of the dusting problem.

The calcined delayed coke employed has the characteristics enumerated above, i. e., real density and particle size distribution.

The calcined fluid coke utilized is prepared by the recently developed fluid coking process, e. g., see Serial No. 375,088, filed August 10, 1953. For completeness the process is supplied in further detail although it should be understood the fluid coking process is no part of this invention.

The fluid coking unit consists basically of a reaction vessel or coker and a heater or burner vessel. In a typical operation the heavy oil to be processed is injected into the reaction vessel containing a dense turbulent fluidized bed of hot inert solid particles, preferably coke particles. A transfer line reactor or staged reactorscan be employed. Uniform temperature exists in the coking bed. Uniform mixing in the bed results in virtually isothermal conditions and efiects instantaneous distribution of the feed stock. In the reaction zone the feed stock is partially vaporized and partially cracked. Product vapors are removed from the coking vessel and sent to a fractionator for the recovery of gas and light distillates therefrom. Any heavy bottoms is usually returned to the coking vessel. The coke produced in the process remains in the bed coated on the solid particles. Stripping steam is injected into the stripper to remove oil from the coke particles prior to the passage of the coke to the burner.

The heat for carrying out the endothermic coking reaction is generated in the burner vessel, usually but not necessarily separate. A stream of coke is thus transferred from the reactor to the burner vessel, such as a transfer line or fluid bed burner, employing a standpipe and riser system; air being supplied to the riser for conveying the solids to the burner. Sufiicient coke or added carbonaceous matter is burned in the burning vessel to bring the solids therein up to a temperature suflicient to maintain the system in heat balance. The burner solids are maintained at a higher temperature than the solids in the reactor. About 5% of coke, based on the feed, is burned for this purpose. This may amount to approximately 15% to 30% of the coke made in the process. The net coke production, which represents the coke made less the coke burned, is withdrawn.

Heavy hydrocarbon oil feeds suitable for the coking process include heavy crudes, atmospheric and crude vacuum bottoms, pitch, asphalt, or heavy hydrocarbon petroleum residua or mixtures thereof. Typically such feeds can have an initial boiling point of about 700 F. or higher, an A. P. I. gravity of about 0 to 20, and a Conradson carbon residue content of about 5 to 40 wt. percent. (As to Conradson carbon residue see A. S. T. M. Test D-18052.)

Further details on the distinctions between fluid coking and delayed coking are given in Oil and Gas Journal, March 22, 1954, pages 126, 127, 130 and 131.

The fluid coke product is laminar in structure and may comprise some 30 to 100 superposed layers of coke. The size distribution is normally such that a predominant portion, i. e., about 90 weight percent has adiameter in the range of about 20 to mesh. The real density of these coke particles after the required calcining is in the range of 1.83 to 1.93, preferably 1.87 to 1.92.

The calcining of the delayed and fluid coke is performed in the conventional manner, i. e., a calcination at a temperature in the range of about 2000 to 2800 F. or higher. This can be done in a fluid, moving or fixed bed in the presence of an atmosphere such as air, nitrogen, carbon dioxide, hydrogen, or by the use of shot. The calcination is conducted until real densities in the specified range are obtained. The time necessary is thus in the range of about 0.5 to hours. Longer calcining times may be used, especially in the lower temperature range, without deleterious effects.

In the manufacture of the electrode itself the coke blend is admixed with and charged together with a carbonaceous binder to the fabrication system. The binders utilized are conventional and include materials such as the aromatic coal tar pitch binders e. g. see U. S. Patent No. 2,683,107. Such binders generally have melting points lying within the range of 70120 C. They contain small amounts of hydrogen (about 5% or less). The concentration of benzene and nitrobenzene insoluble portions represent preferably about to and 5% to 15%, respectively, of the binder. The binder is utilized in an amount of about 18 to parts by weight per 100 parts of coke blend.

In general, two types of electrodes are employed by the industry (a) prebaked and (b) Soderberg self-baking electrodes. In the former, a mixture comprising about 78-82% of calcined coke blend and about 18-22% of coal tar pitch is molded at pressures of about 3000-6500 p. s. i. or extruded, and then baked for periods up to 30 days at l800 to 2400 F. These preformed electrodes are then used in electrolytic cells, being slowly lowered into the molten alumina as they are consumed. Butts of the unconsumed electrodes are reground and used in subsequent electrode preparations. Some green coke can be calcined during the baking operation.

The Soderberg process involves the continuous or intermittent addition of a coke-coal tar pitch paste to the top of the cell as the electrode components in the lower part of the cell are consumed. In this operation the paste represents a blend of about 70% to 72% coke charge and 28% to 30% of pitch. The cells operate usually at temperatures of 1700 to 1900 F. and electrodes are consumed at the rate of about 0.5 to 1.0 inch per day. The paste is baked into an electrode by the hot cell gases in the period between the time it is added at the top and time it is used. The net consumption of coke represents 0.4 to 0.7 lb. per pound of aluminum metal produced. Both methods have in common the baking of the mixed coke charge and binder at a temperature in the range of 1700 to 2400 F.

This invention and its advantages will be better illustrated by the following examples of electrodes prepared in the manner taught.

EXAMPLE 1 Prebaked electrodes were prepared from diiferent calcined coke charging stocks using about 30 parts by weight of coal tar pitch as binder and temperatures of about 1820 F. Further conditions of preparation and test results are given in Table I.

Several points should be noted. The densities of the electrodes prepared from charges containing fluid coke were lower than those made from delayed coke exclusively, and consequently the electrodes themselves less subject to dusting. In terms of resistivity, crushing strength (minimum 4400) and minimum density the electrodes prepared from the blends were completely satisfactory. At the upper limit for fluid coke, however, i. e., wt. percent, the quality becomes marginal, demonstrating that the fluid and delayed cokes are not equivalents.

The detailed particle size distribution in the coke charge to the electrodes prepared from about 30% fluid coke is given in Table II.

Cir

Table II Mesh Size, Wt.

inches Percent Coke coke itself is given A detailed breakdown on the fluid EXAMPLE 2 Additional prebaked electrodes were prepared and the results and further details are presented in Tables IV and V. The data demonstrate again the lower density and satisfactory resistivity and crushing strength obtained 00 from the blends.

Table I EVALUATION OF ELECTRODES CONTAINING FLUID COKE Electrical Resistivity, Crushing Bulk Density, Ohm-in. l0- Molded at- Strength, P. s. l., g./cm. Percent Percent Molded at Fluid Delayed Coke Coke 3,000 4,500 6,000 3,000 6,000 3,000 6,000 p. s. i. p. s. i. p. s. l. p. s. i. p. s. i.

Table IV EVALUATION OF ELECTRODES CONTAINING FLUID COKE Electrical Resistivity, Crushing Strength, Bulk Density, Ohm-in. X 10-, P. s. 1., Molded atg./cm. Percent Percent Molded at- Fluid Delayed Coke Coke 3,000 6,000 3,000 6,000 3,000 6,000 p. s. i. p. s. i. p. s. i. p. s. i.

Table V MESH SIZES OF THE COKE USED IN THE ABOVE ELECTRODES Tyler Mesh, Wt. Percent Type Coke inch +8 100% Delayed 1.3 4 2 2. 2 12. 5 22.1 12.9 3. 3 6. 7 11. 9 22. 4 75% Delayed 1.0 d 1 1. 7 9. 4 16. 7 9. 7 2. 5 5. 0 8. 0 17. 2 Fluid 0.1 0. 4 1. 8 15. 8 6. 2 0. 7

In order to give more details on the preparation of fluid coke, the following conditions of operation of the fluid coker are set forth below.

The advantages of this invention are the elimination of dusting and reduction of heat loss from the cells with the lower density electrodes. The fact that the fluid coke can be used without grinding is an asset. Grinding the fluid coke permits the use of larger quantities in the blend.

It is to be understood that this invention is not limited to the specific examples which have been offered merely as illustrations and that modification may be made without departing from the spirit of the invention.

What is claimed is:

1. In a method of making a carbon electrode by baking coke particles with from 18 to 45 parts by weight of a carbonaceous binder at a temperature in the range of 1700 to 2400" F. the improvement which comprises utilizing as the coke charge to the baking a mixture of calcined fluid coke having a real density in the range of 1.83 to 1.93 and calcined delayed coke particles having a minimum real density of 2, the fluid coke particles being utilized in an amount of from 1 to 50 wt. percent based on the total coke charge and having been produced by contacting a heavy petroleum oil coking charge stock at a coking temperature with a body of fluidized coke particles in a reaction zone wherein the oil is converted to product vapors and carbonaceous solids are continuously deposited on the coke particles, removing product vapors from the coking zone, heating a portion of the coke particles from the coking zone in a heating zone to increase the temperature of said fluidized particles, returning a portion of the heated coke particles from the heating zone to the coking zone and withdrawing coke product particles.

2. The method of claim 1 in which the particle size distribution of the delayed coke is uch that about Weight percent has a diameter in the range of about /2" to 200 mesh and the fluid coke has a particle size distribution such that about weight percent is in the range of 20 to 80 mesh.

References Cited in the file of this patent UNITED STATES PATENTS Shea Aug. 7, 1951 Schutte June 10, 1952 Mattox Ian. 25, 1955 OTHER REFERENCES 

1. IN A METHOD OF MAKING A CARBON ELECTRODE BY BAKING COKE PARTICLES WITH FROM 18 TO 45 PARTS BY WEIGHT OF A CARBONACEOUS BINDER AT A TEMPERATURE IN THE RANGE OF 1700*F. THE IMPROVEMENT WHICH COMPRISES UTILIZING AS THE COKE CHARGE TO THE BAKING A MIXTURE OF CALCINED FLUID COKE HAVING A REAL DENSITY IN THE RANGE OF 1.83 TO 1.93 AND CALCINED DELAYED COKE PARTICLES HAVING A MINIMUM REAL DENSITY OF 2, THE FLUID COKE PARTICLES BEING UTILIZED IN AN AMOUNT OF FROM 1 TO 50 WT. PERCENT BASED ON THE TOTAL COKE CHARGE AND HAVING BEEN PRODUCED BY CONTACTING A HEAVY PETROLEUM OIL COKING CHARGE STOCK AT A COKING TEMPERATURE WITH A BODY OF FLUIDIZED COKE PARTICLES IN A REACTION ZONE WHEREIN THE OIL IS CONVERTED TO PRODUCT VAPORS AND CARBONACEOUS SOLIDS ARE CONTINUOUSLY DEPOSITED ON THE COKE PARTICLES, REMOVING PRODUCT VAPORS FROM THE COKING ZONE, HEATING A PORTION OF THE COKE PARTICLES FROM THE COKING ZONE IN A HEATING ZONE TO INCREASE THE TEMPERATURE OF SAID FLUIDIZED PARTICLES, RETURNING A PORTION OF THE HEATED COKE PARTICLES FROM THE HEATING ZONE TO THE COKING ZONE AND WITHDRAWING COKE PRODUCT PARTICLES. 